TN-S vv TN-C-S

[NoSparks]

I'm interested in collating the differences in the requirements (not the definitions) between TN-S and TN-C-S.

I've aleady seen an example in another post tonight: "TN-S Ze max is 0.8", Any other quotes (with regs reference if possible)?

 

[Des 56]

This sort of thing?
Table 54.8 PME minimum main protective bonding size 10mm

 

[Mac]

17th Edition Incoming Supply

 

[NoSparks]

17th Edition Incoming Supply

Yes Mac, a list like that but giving the different requirements for TN-S and TN-C-S.



Were not getting very many replies in! does that mean that most people are like me and don't know the difference requirements?

 

[Spudnik]

Are there any substantial requirements to meet that hugely differ between the two?

Apart from the Ze of course.

 

[crabapple]

TNCS not to be used in many special locations

 

[Mac]

TNCS not to be used in many special locations

What special locations are you talking about.I have a TNCS at home.I have a swimming pool and a saunaAnd a yacht moored up in the marina at the bottom of my garden

 

[Spudnik]

What special locations are you talking about.I have a TNCS at home.I have a swimming pool and a saunaAnd a yacht moored up in the marina at the bottom of my garden

Then you woke up

 

[Mac]

Gotta dream mate

 

[Yooj]

Not BS7671, but ESQCR...DNO tend not to allow exportation of TNCS earth. Some do allow say two metres outside equipotential zone. Generally, from experience, they seem to be ok with exporting TNS earth.

Yooj

 

[xraytek]

708.411.4= TN-C-S not permitted in caravan parks or similar

709.411.4= TN-C-S not permitted for supply to boat or similar

711.411.4= TN-C-S not permitted for exhibition stands or similar

717.411.4= TN-C-S not permitted for mobile or transportable units

721.411.4= TN-C-S not permitted for supply to Caravans

 

[Mac]

I'm interested in collating the differences in the requirements (not the definitions) between TN-S and TN-C-S.

I've aleady seen an example in another post tonight: "TN-S Ze max is 0.8", Any other quotes (with regs reference if possible)?

TNC-S. As everyone knows, the customer is provided with a connection to the System Neutral to use as their main earth. The Neutral will be earthed at the star point of the supply transformer, again at the end of the main, at or beyond the connection of the furthest customer on that cable. In addition, earths are added to the neutral along the main at appropriate points, to reduce the System Neutral to earth potential under normal conditions, and to limit the potential on the System Neutral compared to earth in the event of a break occurring on the Neutral conductor back towards the supply transformer.

With TNS, the star point of the supply transformer is earthed. A separate conductor is provided back to the supply transformer to connect to the customer's earth transformer. Virtually all TNS supplies are provided on lead mains, the lead sheath / steel armouring effectively acts like a large earth rod, equivalent to the length of the cable.

With TT, the star point of the supply transformer is earthed. No earth connection is provided to the customer

 

[Guest123]

That looks a bit familiar too!^^^


Original Post by me -


Ok mate here goes.

We are all used to TT, TNS and TNC-S as supply types.

I'll start with TNC-S. As everyone knows, the customer is provided with a connection to the System Neutral to use as their main earth. The Neutral will be earthed at the star point of the supply transformer, again at the end of the main, at or beyond the connection of the furthest customer on that cable. In addition, earths are added to the neutral along the main at appropriate points, to reduce the System Neutral to earth potential under normal conditions, and to limit the potential on the System Neutral compared to earth in the event of a break occurring on the Neutral conductor back towards the supply transformer.

With TNS, the star point of the supply transformer is earthed. A separate conductor is provided back to the supply transformer to connect to the customer's earth transformer. Virtually all TNS supplies are provided on lead mains, the lead sheath / steel armouring effectively acts like a large earth rod, equivalent to the length of the cable.

With TT, the star point of the supply transformer is earthed. No earth connection is provided to the customer.

Both have disadvantages TN-S for instance - if the sheathing of the cable breaks down this could go un-noticed for years as the bonding conductors would be providing a paralell path to earth. You would know straight away if a TN-C-S conductor was lost.

Most DNO's stste that anyone on TN-S should be "upgraded to TN-C-S as TN-S is no longer supplied on new installations

Take a look at the link below aswell.


Hope this helps.


http://---------------/Publish/WireR...s_answered.pdf

 

[trebor]

Although you are correct in the basic differences and pros and cons between TN-S and TN-C-S there are inherent dangers with TN-C-S if not correctly used, that is not to be used on restricted installations as as defined by the ESQCR or where correct sized main equipotential bonding is not used or equipotential zones are not established for example in a garden with metal fittings where there may be a potential difference between the neutrally earthed conductive parts and and ground earth.

In the rare circumstances where a service PEN conductor is severed on its own then it is not always obvious in the installation, where bonded steelwork is providing a low resistance earth path this will act as a neutral, this is the reason for the larger csa of main bonding conductors now

In the early 70`s I was an intallations inspector when PME first appeared, 2.5sqmm was the minimum size, this caused more than one property to burn on an estate in Romford when network current tried to use the bonding via metal water services as a neutral after a network neutral was severed on the overhead lines

TN-C-S is used primarily for economic reasons by DNO`s however it does provde a lower Ze than TN-S in most cases

So used properly by good electricians TN_c_s is no problem, used by DIY or our John Wayne friends, not so good

 

[Mac]

That looks a bit familiar too!^^^


Original Post by me -


Ok mate here goes.

We are all used to TT, TNS and TNC-S as supply types.

I'll start with TNC-S. As everyone knows, the customer is provided with a connection to the System Neutral to use as their main earth. The Neutral will be earthed at the star point of the supply transformer, again at the end of the main, at or beyond the connection of the furthest customer on that cable. In addition, earths are added to the neutral along the main at appropriate points, to reduce the System Neutral to earth potential under normal conditions, and to limit the potential on the System Neutral compared to earth in the event of a break occurring on the Neutral conductor back towards the supply transformer.

With TNS, the star point of the supply transformer is earthed. A separate conductor is provided back to the supply transformer to connect to the customer's earth transformer. Virtually all TNS supplies are provided on lead mains, the lead sheath / steel armouring effectively acts like a large earth rod, equivalent to the length of the cable.

With TT, the star point of the supply transformer is earthed. No earth connection is provided to the customer.

Both have disadvantages TN-S for instance - if the sheathing of the cable breaks down this could go un-noticed for years as the bonding conductors would be providing a paralell path to earth. You would know straight away if a TN-C-S conductor was lost.

Most DNO's stste that anyone on TN-S should be "upgraded to TN-C-S as TN-S is no longer supplied on new installations

Take a look at the link below aswell.


Hope this helps.


http://---------------/Publish/WireR...s_answered.pdf

Hi Lenny,must have found it on the same link you did